1. Field of the Invention
The invention relates to an gas lasers such as excimer lasers, and particularly to an adjustably line-narrowed laser system for use with a photolithographic imaging system.
2. Discussion of the Related Art
A line-narrowed excimer or molecular fluorine laser used for microlithography provides an output beam with specified narrow spectral linewidth. It is desired that parameters of this output beam such as wavelength, linewidth, and energy and energy dose stabilty be reliable and consistent. Narrowing of the linewidth is generally achieved through the use of a linewidth narrowing and/or wavelength selection and wavelength tuning module (hereinafter "line-narrowing module") consisting most commonly of prisms, diffraction grating and, in some cases, optical etalons. A line-narrowing module typically functions to disperse incoming light angularly such that light rays of the beam with different wavelengths are reflected at different angles. Only those rays fitting into a certain "acceptance" angle of the resonator undergo further amplification, and eventually contribute to the output of the laser system.
Depending on the type and extent of line-narrowing and/or selection and tuning that is desired, and the particular laser that the wave-length selector is to be installed into, there are many alternative linenarrowing configurations that may be used. For this purpose, those shown in U.S. Pat. Nos. 4,399,540, 4,905,243, 5,226,050, 5,559,816, 5,659,419, 5,663,973, 5,761,236, and 5,946,337, and U.S. patent applications Ser. Nos. 09/317,695, 09/130,277, 09/244,554, 09/317,527, 09/073,070, 60/124,241, 60/140,532, and 60/140,531, each of which is assigned to the same assignee as the present application, and U.S. Pat. Nos. 5,095,492, 5,684,822, 5,835,520, 5,852,627, 5,856,991, 5,898,725, 5,901,163, 5,917,849, 5,970,082, 5,404,366, 4,975,919, 5,142,543, 5,596,596, 5,802,094, 4,856,018, and 4,829,536, are each hereby incorporated by reference into the present application.
Depending on the extent of line-narrowing that is desired, excimer laser systems can be broadly classified into three general groups: broad-band, semi-narrow band and narrow-band. A fourth classification, very narrow-band, is sometimes referred to when it is desired to distinguish those lasers in the narrow-band group that have a particularly very narrow output emission bandwidth. Broadband excimer lasers do not have any line narrowing unit or components. Therefore, the relatively broad (i.e., 300 pm) characteristic output emission bandwidth of a KrF or ArF laser, e.g., is outcoupled from the laser resonator.
A semi-narrow band laser has a characteristic output that is narrowed using most typically prisms. The output emission bandwidth of the semi-narrowed laser is reduced for a KrF or ArF laser from around 300 pm to less than 100 pm. The semi-narrow band laser may be used in combination with catadioptic (reflective) optical imaging systems for industrial photolithography.
A narrow band laser typically further includes a grating such that the line-narrowing unit comprises a littrow configuration of prisms and a grating. One or more etalons may also be added for further line-narrowing, and there are other techniques described in the patents and patent applications referenced above. Such techniques can narrow the linewidth to below 1 pm. As such, narrow band lasers are used in combination with refractive optical imaging systems.
Typical line-narrowing modules suffer from a common limitation. That is, the bandwidth is fixed, and can only be adjusted by substituting a different line-narrowing module into the laser resonator having optical components with different beam expanding and/or dispersive properties. Alternatively, one or more components of an existing line-narrowing module may be replaced. In either case, the bandwidth of the laser having the conventional line-narrowing unit may not be adjusted in real-time, i.e., without replacing or disassembling the line-narrowing unit.
FIG. 1 schematically illustrates a conventional resonator design for a line-narrowed excimer or molecular fluorine laser. The resonator design shown in FIG. 1 includes a gain medium 1, an output coupler 2, a beam expanding prism 3, a dispersive prism 4 and a resonator reflector 5. The resonator reflector 5 may be a highly reflective mirror in which case the resonator design is for a semi-narrow band laser. The resonator reflector 5 may also be a dispersive prism in which case the resonator design is for a narrow band laser.
The output emission linewidth of a laser having a resonator design as shown in FIG. 1 is fixed and determined by the beam expanding power of the beam expander 3 and the dispersive power of the dispersive element 4 (and also the dispersive power of the grating if one is used). To change the bandwidth of a laser whose resonator configuration is schematically illustrated at FIG. 1, either the prism material, the dispersion or the geometry of the prisms 3, 4 would be changed. This means replacing already installed prisms with new ones having the different refractive index or geometrical properties, or replacing the entire line-narrowing module of the laser. It is desired to have a resonator design wherein the bandwidth may be adjusted without replacing components of an installed line-narrowing unit or the entire line-narrowing unit itself.
One technique for adjusting a laser linewidth without replacing the line-narrowing unit itself or components of an installed line-narrowing unit, is described at U.S. patent application Ser. No. 09/244,554 incorporated by reference above. In the '554 application, a beam expander has an adjustable magnification which in turn alters the dispersive power of the dispersion element. A preferred embodiment describes a pair of prisms that are synchronously rotatable for adjusting the magnification of the beam expander, and a mirror or grating is tilted for adjusting the wavelength.
Another technique for performing linewidth adjustment is described at U.S. patent application Ser. No. 09/317,527, also incorporated by reference above. In the '527 application, a pressure tuned etalon is provided. The pressure of the gaseous volume between the plates of the etalon, or the spacing between the plates, is adjusted. By changing the pressure in the former step, the index of refraction is thereby changed. Either way the free spectral range of the etalon is changed as the pressure is tuned. The wavelength and linewidth may be adjusted in this way. However, the techniques set forth in the '527 application have a more advantageous use for line selection and wavelength tuning, than for adjusting the linewidth of a laser.